The present invention relates in general to a delayed coking process, and more particularly, to an apparatus and method for maximizing the yield of petroleum products in a delayed coking process.
Essentially, delayed coking is a thermal process whereby reduced crude, or petroleum residue, is rapidly heated in a coker furnace and then confined in one of a pair of reaction zones, or coke drums, under proper conditions of temperature and pressure until the portion of the crude not vaporized in the furnace is converted to vapor and coke. In the coking process, before the reduced crude is rapidly heated and directed to the coke drum, it is preheated and fed into a coker fractionating tower, or fractionator, which is in fluid communication with the coke drum. The petroleum residue is fed into a lower, surge zone at the bottom of the fractionator, where it mixes with recycle liquid, which is condensed from product vapors in the fractionator, to define a pool of liquid at the bottom of the fractionator. The mixture of petroleum residue and recycle liquid is fed to the coker furnace, where its temperature is raised to the level necessary for coke formation in the coke drums. The heated mixture is pumped to one of the coke drums, where it is converted to coke and light hydrocarbon product vapors. The product vapors pass upward through the coke drum, leave overhead and flow back to the fractionator, where they enter in a flash zone, which is above the surge zone containing the mixture of petroleum residue and recycle liquid. The product vapors are quenched and washed as they rise through a wash zone in a tortuous path defined by a series of baffles and wash trays contacted with pumped-back hot gas oil. The washing operation just described cleans and cools the product vapors and simultaneously condenses a portion of the product vapors into the liquid recycle which falls to the bottom of the fractionator. The remaining product vapors continue to rise into the upper portions of the fractionator, where they are taken off as heavy gas oil, light gas oil, gasoline and gas.
It would be ideal for all of the product vapors to pass undiminished through the wash zone and be taken off as product, but condensation of some of the product vapors into the liquid recycle is an inherent characteristic of the washing operation. In addition, as the condensed recycle falls from the wash zone and through the flash zone, it comes into direct contact with the rising product vapors. As a result, an additional portion of the product vapors condenses before the vapors reach the wash zone. Furthermore, the pool of petroleum residue and recycle liquid is relatively cool with respect to product vapors in the flash zone, which is just above the pool. Therefore, the coolness of the pool causes a portion of the vapors in the flash zone to condense and fall into the pool. Moreover, as the newly condensed recycle liquid falls into the pool, it causes liquid to splash up into contact with the product vapors in the flash zone, resulting in still further condensation of the vapors. Therefore, the product vapor flow through the fractionator is reduced and the product yield of the fractionator is reduced. In addition, greater amounts of recycle liquid are formed and must be reheated in the coker furnace.